The iron in a sample is in the form of an iron(II) compound,
which is colorless because Fe2+ ions have zero molar absorptivity for visible light.
Thus to determine the concentration of Fe2+ ions in solution using a visible wavelength of light,
the ions must react quantitatively with other species to form colored compounds.
Fe2+ ions react with o-phenanthroline molecules, C12H8N2, to form complex of orange-red [Fe(o-phen)3]2+ ions